Thank you for clarifying. Unfortunately, there is no possibility to trigger business rules when membership in a rule-based group is updated based on the specified criteria. As a solution, you can use a scheduled task that will do the following:
- Send email notifications to group members and mark the users (e.g. by setting a custom Boolean attribute to True).
- If a user is marked and is no longer a member of the group, the mark will be cleared.
Finally, the scheduled task will look like the following: